An image sensor is a semiconductor device that transforms a light signal input from outside to an electric signal (i.e., performs photoelectric transformations) to provide image information corresponding to the light signal. Recently, a three-dimensional (3D) image sensor that provides distance information as well as the image information based on the light signal has been suggested. Typically, the 3D image sensor may measure a distance between each unit pixel and an object by measuring a time of flight (TOF) that corresponds to an elapsed time during which an optical pulse (i.e., a laser pulse) is emitted toward the object by a light source, and then input to the unit pixel after being reflected by the object.